This article details lenses for single-lens reflex and digital single-lens reflex cameras (SLRs and DSLRs respectively). The emphasis is on modern lenses for 35 mm film SLRs and for "full-frame" DSLRs with sensor sizes less than or equal to 35 mm.
Design
Interchangeable lenses
On most SLR and DSLR cameras, the lens can be changed. This enables the use of lenses that are best suited for a given photographic need, and allows for the attachment of specialized lenses. Film SLR cameras became the dominant option for professionals and enthusiasts since the late 1950s, and over the years, millions of lenses have been produced, both by camera manufacturers (who typically only make lenses intended for their own camera bodies) and by third-party optics companies who may make lenses for several different camera lines.
DSLRs were introduced in the early 1990s, and the first affordable DSLRs were introduced about ten years later, allowing those types to become the most popular interchangeable-lens option for digital photography. Some manufacturers, such as Minolta, Canon and Nikon, chose to make their DSLRs 100% compatible with their existing SLR lenses from the beginning, allowing owners of new DSLRs to continue to use their existing lenses and get a longer lifespan from their investment. Others, for example Olympus, opted for new lens mounts and series of lenses for their DSLRs. Modern Pentax DSLRs are backwards-compatible with K-mount SLR lenses, though features that are included in newer lenses (e.g. autofocus) may not work. There are a few exceptions; the MZ and ZX series of Pentax film cameras do not work with some older lenses.
In general, lenses are only directly interchangeable within the mount system for which they are built. Using a lens intended for one mounting system on a camera body with a different system requires an adapter, which can result in compromises such as loss of functionality (e.g. lack of autofocus or automatic aperture control). Furthermore, the adapter may require an additional optical element to correct the optical design, as the lens mount specifies the flange focal distance (FFD), which is the distance from the mount interface to the focal plane on the image sensor or film. In general, camera bodies with shorter FFDs permit the creation of simple mechanical adapters without additional optics; adapters may not be available to bridge every combination of lens mount and camera mount.
Aperture and depth of field
The aperture of a lens is the opening that regulates the amount of light that passes through the lens. It is controlled by a diaphragm inside the lens, which is in turn controlled either manually or by the exposure circuitry in the camera body.
The relative aperture is specified as an f-number, the ratio of the lens focal length to the diameter of its effective aperture. A small f-number like 2.0 indicates a large aperture (more light passes through), while a large f-number like 22 indicates a small aperture (little light passes through). Aperture settings are usually not continuously variable; instead, the diaphragm has typically 5–10 discrete settings. The normal "full-stop" f-number scale for modern lenses represents a halving of the light admitted by the preceding stop; since the square of the diameter is directly proportional to the area (light admitted), the scale is as follows: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, but many lenses also allow setting it to half-stop or third-stop increments. A "slow" lens (one that is not capable of passing a lot of light through) might have a maximum aperture from 5.6 to 11, while a "fast" lens (one that can pass more light through) might have a maximum aperture from 1 to 4. Fast lenses are, by definition, larger than slow lenses (for comparable focal length), and typically cost more.
The aperture affects not only the amount of light that passes through the lens, but also the depth of field of the resulting image: a larger aperture (a smaller f-number, e.g. ) will have a shallow depth of field, while a smaller aperture (a larger f-number, e.g. ) will have a greater depth of field.
Focal length and angle of view
The focal length of a lens, together with the size of the image sensor or film in the camera, determines the angle of view. A lens is considered to be a "normal lens" in terms of its angle of view on a camera when its focal length is approximately equal to the diagonal dimension of the film format or image sensor format. This diagonal angle of view—about 53 degrees—is often said to approximate the angle of view of the human eye; since the angle of view of a human eye is at least 140 degrees, it is sometimes noted as "similar to the angle of crisp human vision." A wide-angle lens has a shorter focal length and includes more of the viewed scene than a normal lens; a telephoto lens has a longer focal length and captures a small portion of the scene, making it seem closer.
Lenses are not labeled or sold according to their angle of view, but rather by their focal length, usually expressed in millimeters. However, this specification is insufficient to compare lenses for different cameras, because field of view also depends on sensor size. For example, a 50 mm lens mounted on a Nikon D3 (a full-frame camera) provides approximately the same field of view as a 32 mm lens mounted on a Sony α100 (an APS-C camera). Conversely, the same lens can produce different fields of view when mounted on different cameras. For example, a 35 mm lens mounted on a full-frame Canon EOS 5D provides a slightly wide-angle view, while the same lens mounted on an APS-C Canon EOS 400D provides a "normal" or slightly telephoto view.
In order to more easily compare lens–camera pairs, it is common to use their 35 mm equivalent focal length. For example, when talking about a 14 mm lens for a Four Thirds system camera, one would not only indicate that it has a focal length of 14 mm, but also that its "35 mm equivalent focal length" is 28 mm. This way of talking about lenses is not just limited to SLR and DSLR lenses; it is very common to see this focal length equivalency in the specification of the lens on a digicam.
Table of formats and angle of view Values in the following table are approximate, and apply to rectilinear lenses only, not to fisheye lenses.
+Common camera image sensor formats and rectilinear angles of view Sensor size (W×H) 120 film (Medium format) to 35 mm or full-frame APS-H APS-C (typ) Foveon X3The crop factor in this column refers to the original Foveon X3 sensor. Current versions of the Foveon X3 are standard APS-C size with a 1.5 crop factor. Four Thirds 1" Crop factor to to Lens type Angle of view(deg. diagonal) Focal length(mm) Ultra wide angle 118° to 111° to 100° to 92° to Wide Angle 84° to 75° to 63° to Normal 57° to 51° to 46° to 43° to Short Tele 29° to 23° to Telephoto 18° to 14° to 12.5° to Super telephoto 8° to 6° to 5° to Ultra telephoto 4° to 3° to 2° to Some cameras usingthis sensor size: 120 mm filmFuji GX680Hasselblad 503CWMamiya 645Pentax 645NII 35 mm filmCanon EOS 6DNikon D5, D810Pentax K-1 IISony α7 IV Canon EOS-1DLeica M8 APS filmCanon EOS 90DNikon D7200Pentax K-3 Mark IISony α6500 Sigma SD14Sigma DP2 Leica Digilux 3Olympus E-3Olympus E-30Olympus E-620Panasonic DMC-G1 Nikon CXSony RX100Samsung NX mini
Types of lenses
Zoom lenses
The focal length of a zoom lens is not fixed; instead it can be varied between a specified minimum and maximum value. Modern lens technology is such that the loss of image quality in zoom lenses (relative to non-zoom lenses) is minimal, and zoom lenses have become the standard lenses for SLRs and DSLRs. This is different from the late 1980s when, due to image quality concerns, most professional photographers still relied primarily on standard non-zoom lenses. However, zoom lenses still typically have a lower maximum aperture than fixed-focal ("prime") lenses for the same weight and cost, especially for shorter focal lengths.
Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or "4×" zoom. Typical zoom lenses cover a 3.5× range, for example 24–90 mm (standard zoom) or 60–200 mm (telephoto zoom). "Super-zoom" lenses with a range of 10× or even 14× are becoming more common, although the image quality does typically suffer a bit compared with the more traditional zooms.
The maximum aperture for a zoom lens may be the same (constant) for all focal lengths, but it is more common that the maximum aperture is greater at the wide-angle end than at the telephoto end of the zoom range. For example, a 100 mm to 400 mm lens may have a maximum aperture of f/4.0 at the 100 mm end but will diminish to only f/5.6 at the 400 mm end of the zoom range. Zoom lenses with constant maximum apertures (such as f/2.8 for a 24–70mm lens) are usually reserved for lenses with higher build quality and are thus more expensive than those with variable maximum apertures.
Macro lenses
Macro lenses are designed for extreme closeup work. Such lenses are popular for nature shooting such as small flowers, as well as for many technical applications. As most of these lenses can also focus to infinity and tend to be quite sharp, many are used as general-purpose optics.
Special purpose lenses
Most users of SLR and DSLR cameras stick to using zoom lenses, while a few of the more adventurous amateurs and many professional photographers also invest in a few prime lenses. Special purpose lenses are, as the designation implies, for special purposes, and are not so common.
There are many different kinds of special purpose lenses, the most popular being fisheye lenses, which are extreme wide-angle lenses with an angle of view of up to 180 degrees or more, with very noticeable (sometimes intended) distortion. Perspective control lenses and soft-focus lenses, were more popular with film SLRs but are less popular for DSLRs because the same or similar results can be obtained with post-processing software.
Types of lens mounts
There is almost no commonality between different camera makers regarding lens mount systems. Each manufacturer has developed their own system, and build camera bodies and lenses that only work with their own lens mount, with the Four Thirds System being a partial exception. This was different before 1970 when most of the manufacturers use either M42 or M39 lenses, most of which can still be used depending on the particular adapter you can find.
This does not necessarily mean that one is limited to only mounting, for example, Pentax lenses on a Pentax camera body. There are independent optics companies that make lenses for the various otherwise proprietary mount systems, thus providing alternative sources for lenses that are often of equal quality and/or less expensive than the camera maker's own lenses. Another possibility is the use of adaptors that allow mounting a lens for one system on a camera with a different lens mount. However, the use of an adaptor usually results in reduced functionality, typically requiring the manual setting of aperture and focus, or perhaps not being able to use any aperture other than "wide open".
M42 lens mount The M42 lens mount has been used by Leica, Nikon, Pentax, Canon, Zenit, Praktica, Fujica, Cosina,